It is well known that maximum activity of the Group VIII noble metals for hydrogenation reactions depends upon maintaining the metal in a finely divided state such that there is a maximum ratio of surface area to mass. Perhaps the most common method of achieving a high degree of dispersion involves impregnating salts of the Group VIII noble metals upon porous solid supports, followed by drying and decomposing of the impregnated salt. On non-zeolitic supports, the drying and calcining operations often bring about a substantial migration and agglomeration of the impregnated metal, with resultant reduction in activity. In more recent years, with the advent of highly active crystalline zeolite catalysts of the aluminosilicate type, it has become common practice to ion-exchange the desired metal salt into the zeolite structure in an attempt to achieve an initial ionic bond between each metal atom and an exchange site on the zeolite, thus achieving the ultimate in dispersion of metal while also bonding the metal to the zeolite in such manner as to minimize migration and agglomeration during the drying and calcining steps in which at least a portion of the metal is oxidized and converted to a non-zeolitic form. This ion exchange technique is particularly desirable in the case of dual-function catalysts such as hydrocracking catalysts wherein it is desirable to maintain an active hydrogenating site closely adjacent to an acid cracking site. These efforts have met with varying degrees of success.
Even though the above described ion-exchange techniques can give a high degree of initial dispersion of the Group VIII noble metal on the support, conditions encountered during subsequent use of the catalyst may bring about a maldistribution of the metal with resultant reduction in activity, entirely independent of normal deactivating phenomena such as coking, fouling, poisoning, etc. Overheating, or contact with excessive partial pressures of water vapor at high temperatures, such as may occur during oxidative regeneration of the catalyst or during prolonged contacting with hydrocarbon feedstocks, may bring about migration of the active metal away from the exchange sites, and this migration may, under particularly severe conditions, ultimately result in macro-agglomeration of the metal into crystallites of 100-200 A or more in diameter. This particular type of damage is most apt to occur under oxidizing conditions at temperatures of 500.degree.-950.degree. F. where high partial pressures of water vapor are present.
The process of this invention is particularly directed to correcting non-zeolitic Group VIII noble metal maldistribution resulting from thermal and/or hydrothermal stresses encountered by the catalyst in normal usage, regeneration, or during accidental upsets entailing uncontrolled temperatures and/or water vapor partial pressures. Normally these stresses bring about a maldistribution of active metal short of extensive agglomeration to particle sizes larger than about 50 A. For example, metal atoms or aggregates initially located closely adjacent to active exchange sites on the carrier may migrate to other less active areas, thus reducing the statistical likelihood of conjoint action on the feedstock molecules of both an acidic cracking site and a hydrogenation site. Further migration may tend to drive the metal deeper into the support structure, or into pore structures which are relatively inaccessible by feed molecules, all resulting in reduced overall hydrogenation activity.
Limited migration of these types may occur when the catalyst, in a sulfided condition (as e.g., in normal use for hydrocracking), or in an oxidized state (as during regeneration), comes into contact for more than about 30 minutes with water vapor of greater than about 3 psi partial pressure at temperatures above about 500.degree. F. Extended contacting under these conditions, or at extremely high partial pressures of water vapor, e.g., above about 100 psi, can ultimately lead to macroagglomeration of the type previously described. If this should occur, the rejuvenation procedure of this invention is in some cases less effective per se, but can in any case be advantageously utilized following partial redispersal of the agglomerated metal by, for example, the methods described in U.S. Pat. Nos. 3,197,399 and/or 3,287,257. The processes described in these patents, involving respectively alternating oxidation-reduction cycles, and alternating sulfiding-oxidation cycles, can bring about a substantial redispersion of agglomerated metal into particles of less than about 50 A diameter, but do not in most instances bring about a complete recovery of the fresh catalyst activity. The process of this invention is designed to achieve at least a complete recovery of fresh activity; but in nearly all cases it is found that the rejuvenated catalysts actually exhibit greater than fresh activity.
In the case of catalysts which originally contained a difficulty reducible zeolitic monovalent and/or divalent metal such as sodium, calcium, magnesium, nickel, manganese or the like, it has been found that the above described conditions encountered during use of the catalyst also appear to bring about, in addition to migration of the non-zeolitic hydrogenating metal, a detrimental redistribution of the zeolitic metal cations. Residual zeolitic metal cations, particularly sodium, are believed to occupy mainly the relatively unavailable exchange sites in the hexagonal prisms and sodalite cages of the original zeolite structure, but under the described conditions of use, migration to more active cracking sites appears to occur with resultant loss in cracking activity. Divalent metal cations such as the alkaline earth metals, which may have been originally exchanged into the zeolite to achieve hydrothermal stability, may also migrate to undesirable sites. It is hence desirable in the case of these damaged catalysts to remove at least some of the zeolitic mono- and/or divalent metal cations, in addition to redistributing the non-zeolitic Group VIII noble metal hydrogenating component. These are the major objectives of the present invention.
As employed herein, the term "non-zeolitic metal" refers to the metal content of the catalyst, other than anionic lattice metals such as aluminum, which is not chemically bonded to the anionic exchange sites of the zeolite, while conversely, "zeolitic metal" refers to the metal content which is so bonded. The easily reducible metals such as the Group VIII noble metals are normally present primarily as non-zeolitic metal, as a result of previous reduction with hydrogen, oxidation and/or sulfiding treatments. The difficulty reducible metals such as the alkali and alkaline earth metals are normally present almost exclusively as zeolitic cations, since they are not affected by the usual reduction, oxidation or sulfiding treatments. Metals of intermediate reducibility such as nickel, copper and the like may be present in both zeolitic and non-zeolitic form.
In its broadest aspect, the rejuvenation procedure of this invention involves simply digesting the damaged zeolite catalyst with an aqueous ammonia solution, with time and temperature conditions adjusted to effect replacement of at least a portion of the detrimental zeolitic mono- and/or divalent metal cations with ammonium ions, thereby increasing the cracking activity of the catalyst. At the same time, the non-zeolitic Group VIII noble metal content appears to become redistributed in some unknown manner, thereby increasing the hydrogenation activity.
A surprising aspect of the invention is that the aqueous ammonia solution does not extract any significant amount of the Group VIII noble metal from the catalyst. In U.S. Pat. No. 3,899,441 to Hansford, a progenitor rejuvenation process is disclosed, which involves treating the damaged catalyst with ammonia and water vapor under controlled conditions of hydration. This treatment was found to be sufficient to effect desirable redistribution of the Group VIII noble metal, resulting in a substantial recovery of activity, but was not conceived or disclosed as a method for the concomitant removal of zeolitic metal cations. It was believed at the time that the presence of excess aqueous ammonia would tend to solubilize the Group VIII noble metal as amminohydroxide which would then be leached out of the catalyst. It hence came as a distinct surprise to find that large excesses of aqueous ammonia could be utilized at high temperatures and for extended periods of time, sufficient to exchange out substantial proportions of the zeolitic metal cations, while extracting substantially none of the non-zeolitic Group VIII noble metal. The original theory as to the mechanism by which the Group VIII noble metal is redistributed on the zeolite by the ammonia treatment may hence be incorrect; if a soluble species of the Group VIII noble metal is formed, it is apparently so highly basic that it is retained substantially quantitatively in the acid zeolite structure even in the presence of large excesses of aqueous ammonia.
The foregoing is in sharp contrast to the results observed when catalysts containing an iron-group metal are subjected to treatment with the ammoniacal solutions employed herein. A nickel-containing zeolite catalyst for example was found to lose a substantial portion of its nickel content to an ammoniacal ammonium nitrate treating solution. Aqueous ammonium hydroxide alone is an even more efficient extractant for the iron, cobalt or nickel components of regenerated catalysts.
A preferred modification of the invention consists in adding to the ammonia solution a soluble ammonium salt, e.g., ammonium nitrate. With aqueous ammonia alone, it is difficult to exchange out a major proportion of the zeolitic mono- and/or divalent metals without resorting to an expensive multi-stage operation employing large volumes of solution. By adding an ammonium salt the exchange is much facilitated so that larger proportions of the zeolitic metals can be exchanged out at lower temperatures in shorter periods of time, and using smaller volumes of solution. However, in using the added ammonium salt, the conditions of temperature and contact time must be suitably controlled because it is found that, in contrast to aqueous ammonia, the ammonium salts do tend to bring about a solubilization and leaching out of the Group VIII noble metal from the catalyst. This tendency can however be substantially avoided by controlling temperature and contact time.
While the use of an ammonium salt is normally preferred for the foregoing reasons, it will be understood that the use of ammonia alone does present the advantageous feature of eliminating the necessity for careful control of temperatures, contact times, etc. in order to avoid leaching out the Group VIII noble metal. In some types of operations, this feature may outweigh the advantages enumerated above for the use of ammonium salts.
Another progenitor of the present invention is disclosed in U.S. Pat. No. 3,692,692 involving the sequential treatment of the damaged zeolites with aqueous solutions of ammonium salts, and with ammonia under the controlled conditions of hydration disclosed in the above noted U.S. Pat. No. 3,899,441. The preferred techniques disclosed in U.S. Pat. No. 3,692,692 achieve the same basic objectives as herein, and in some cases appear to produce a somewhat more active rejuvenated catalyst. However, the sequential treatment is considerably more expensive and time consuming, and the present single-stage process hence represents a substantial economic advantage. The procedure described herein is simple and economical and gives complete rejuvenation of zeolite-based Group VIII noble metal catalysts wherein a maldistribution of metals has occurred as a result of overheating, or of contracting the catalyst while in an oxidized or sulfided state with water vapor at temperatures between about 500.degree. and 1200.degree. F.